Dropbox hasn't dropped AWS, they moved things off AWS as it made sense to. The article is talking about two things, the move of file storage and a network backbone. Neither of which were done recently.
The file storage move from S3 to Magic Pocket is detailed in these blog posts:
The file storage move from S3 to Magic Pocket is detailed in these blog posts:
https://blogs.dropbox.com/tech/2016/03/magic-pocket-infrastr...
https://blogs.dropbox.com/tech/2016/05/inside-the-magic-pock...
https://blogs.dropbox.com/tech/2016/07/pocket-watch/
The network backbone is talked about here:
https://blogs.dropbox.com/tech/2017/09/infrastructure-update...